Fragile sites are constrictions on metaphase chromosomes, which give the appearance of structural instability or fragility. Fragile sites are expressed only when the culture media of the cell cultures contains specific chemicals that interact with the chromatin as it condenses during the metaphase stage of the cell cycle. Some fragile sites are found in all people and these are referred to as common fragile sites. Other fragile sites are expressed only in certain individuals and their relatives and are referred to as rare fragile sites. The biological significance of fragile sites is not well understood. They seem to reflect some unusual aspect of DNA composition, which causes instability during condensation under laboratory conditions. There are a number of different chemicals that induce fragile sites including aphidicolin, distamycin A, bromodeoxyuridine, methotrexate, low folic acid concentration, and fluorodeoxyuridine, and thus, another means of classifying these sites is according...